rebase: use a better reflog message

rebase: use a better reflog message

Now that the "checkout" invoked internally from "rebase" knows to
honor GIT_REFLOG_ACTION, we can start to use it to write a better
reflog message when "rebase anotherbranch", "rebase --onto branch",
etc. internally checks out the new fork point.  We will write:

  rebase: checkout master

instead of the old

  rebase

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>